The third edition of the Advanced Dungeons & Dragons game, albeit dropping "advanced" from the title to make it more accessible to new players. Though initially published in 2000 by Wizards of the Coast (as the version commonly referred to as "3e"), a "3.5" revision was published in 2003 with some significant changes, and it is the 3.5 revision that most 3rd edition players use. Wizards then went on to release 4e, which received a controversial greeting, and inspired Paizo to monetize the ensuing salt by releasing its own 3.5 with blackjack and hookers, called Pathfinder.

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3rd edition introduced the now ubiquitous "d20 System", where almost every action with a chance of failure is resolved by rolling a d20, applying relevant modifiers, and comparing the result to a set difficulty (or, in some cases, another character's roll) to determine success, referred to by the system as the "Core Mechanic". For example, a fighter attempting to hit a monster with his sword rolls a d20 and may add his Base Attack Bonus, Strength bonus, relevant Weapon Focus bonuses, magical enhancements, etc. with the objective of beating his opponent's Armor Class. Rolling equal to or over the target's AC means he has successfully hit and gets to deal damage. In a similar vein, a rogue attempting to pick a lock rolls a d20 and adds his skill ranks, dexterity bonus, any relevant skill bonuses from feats, modifiers depending on the quality of his equipment, etc. in an attempt to beat the target DC (Difficulty Class) of the lock.

This was generally regarded as a significant improvement on the systems used in 1e and 2e, where many different parts of the game were governed by vastly different mechanics. Restructuring the game around the single core mechanic made gameplay much simpler and easier to pick up for new players.

Characters and creatures in the system are structured around Hit Dice and ability scores, wherein bonuses and traits from various hit dice are stacked together and combined with modifiers derived from the base ability scores to determine the other statistics of the entity. For example, a 2nd level Cleric/3rd level Fighter would have a +1 BAB for his two cleric hit dice and a +3 BAB for his three fighter hit dice, combining to give him a total Base Attack Bonus of +4, which would then be modified by other abilities such as strength or dexterity to determine his overall bonus when making an attack. The hit points granted to him by each of those hit dice would be added together and modified by the constitution score to determine his overall hit point total, and so on.

Almost all entities have six ability scores: Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma - that describe basic qualities of their character. The human average ability score, as the baseline from which all other ability scores are referenced, is 10 or 11. Ability scores higher than this grant bonuses to their relevant checks, and lower than this impose penalties. Every two points of score results in a +1 modifier, such that a score of 8-9 is a -1 penalty, 10-11 is +0, 12-13 is +1, and so on. Different races generally have bonuses and penalties to some ability scores to represent how they differ to humans; for example, graceful but frail elves have a +2 to their Dexterity (giving them an extra +1 bonus to Dexterity-linked checks) but a -2 penalty to their Constitution, whereas stout but surly dwarves receive a +2 to their Constitution but suffer a -2 to their Charisma.

In general, having any ability score reduced to 0 (by magic or other effects) results in incapacitation or death; a 0 Str or Dex character is unable to move himself, a 0 Con character is dead, and a 0 in a mental ability stat results in a coma. Some entities are lacking certain abilities entirely, a situation explicitly different from having a 0 in the stat: for example, a mindless magical construct that cannot think for itself both has no constitution score, as it is not a living being and is not subject to poisons, diseases, and other such things as living beings are; it also has no intelligence score, as it is generally incapable of making its own decisions and instead acts only on the orders given to it by its master.

Everything also fits into size categories, which describe how big or small they are. In ascending order, the categories are Fine, Diminutive, Tiny, Small, Medium, Large, Huge, Gargantuan, and Colossal. A creature's size category modifies its Armor Class and attack bonus (a target relatively larger than you is easier to hit), and determines the damage of its natural weapons and its space and reach. Don't think of your space as the area you occupy, otherwise you start thinking of ten-foot-wide horses; rather, think of it as the area you control. How close do you want to get to a guy swinging a longsword, anyway?

Eberron - called "dungeonpunk", and winner of the "make a new setting for D&D" contest. It made your RPG more JRPG. Seriously: robot player characters, magick trains, and dragon tatoo materia. But maybe you'll like it.

WotC heard about this "open source" thing, and thought they'd get on the bandwagon with Open Gaming License. Players had been making house rules for forever and a day, but WotC riffed off the GNU Public License and wrote some rules where anyone could publish supporting material off the core rules, for free, just acknowledge the source and use the same license so people can make splatbooks for your stuff. The amount of non-WotC material written for 3rd edition skyrocketed, and the d20 System became the heart of dozens of role-playing games in dozens of genres. WotC didn't see royalty checks, but it helped cement their grip on the roleplay game industry during the 3e era and sold a lot of corebooks.

WotC chickened out in the next edition, offering a "new and improved" licensing scheme for it, which is more restrictive and far less used.

Since D&D is relatively mainstream and has been around for so long, many of the design benefits in D&D 3e have been incorporated into other RPGs, so newfags will take these benefits for granted. Keep in mind that D&D 3e broke new ground in many areas, or brought good ideas into widespread attention, and these same newfags probably don't remember how everyone lost their shit when Dragonlance came out.

It just works, bitches. Proof by example: if it wasn't so good over ten years later, people would've jumped ship to Warhammer Fantasy or still be playing AD&D 2nd Ed.

The Open Gaming License allowed for an explosion of peer-created content. Not all of it is good, but you don't have to buy the crap, you can just take the cream. No more "compatible with most fantasy rpgs (wink wink)," and no more small press crap because real publishers are too scared of lawyers.

All task resolution is normalized to a single d20 + relevant modifiers vs. Difficulty Class roll.

Your chance to hit is no longer dependent on an unwieldy THAC0 chart. Each class grants a Base Attack Bonus progression that adds to all your attacks.

The five source-based saving throw classes based on source are reduced to three defense-based saves: Fortitude (I'm a tough guy), Reflex (I'm a nimble guy), and Will (I'm a wiseguy, er, wise guy).

Skills replace nonweapon proficiencies. Each class has a set of skills that they excel in, though any character can take any skill they want. The designers finally realized that a fighter shouldn't have to stand at the bottom of a wall and reach ineffectually just because "Climb Walls" wasn't in his class features.

The Feats system brought in awesome customizing of classes. You want a swordfighter that specializes in sabre-&-dirk fighting? How about a gladiator that brawls unarmed and can go toe-to-toe with a stone golem? We got you covered. It's like a DIY kit for class features.

While the older editions covered the basic realistic combat styles, 3e started to see just how far "heroic" combat can go.

Class restrictions based on race are gone. Anybody can take any class to any level. You no longer need minimum attributes to take a core class; instead, those attributes contribute to the power of the class.

Prestige classes are introduced, which are more specialized classes often associated with organizations and the like. They usually require skills and feats, with the design philosophy that specific class levels should never be prerequisites for a prestige class.

Rules for monsters as player races. With the "Effective Class Levels" system, your party really can have a rakshasa or doppelganger in it and it will balance out, at least in theory. Nearly everything in the Monster Manual has a level adjustment score for use.

Exceedingly easy and rewarding to make homebrew content for, it's a versatile and open system.

Attempted to fix it with the nigh-endless train of prestige classes in the "Complete BLAH" and "Book of Vile/Exalted BLAH" and Tome of Battle: 9 Euphemisms For My Dick... Starting with 6 prestige classes in the 3.0 DMG (16 in the 3.5 DMG), there's 120 more in official splatbooks (349 in the official 3.5 splatbooks), and that's not counting the Epic prestige classes above level 20, or prestige classes introduced in modules, and I haven't even started on the prestige classes mentioned in official settings...

Race/class restrictions may be gone, but this edition has merely swapped open restriction of class paths for seemingly offering open choice of character paths yet penalising builds that don't play to your race's strengths. Race and class selection still favours humans heavily; each race has a "favoured class" that is considered the archetype for them, and any difference in levels of 2 or greater between classes when one is not the FC has the game impose increasingly severe XP penalties. Humans have a rule that lets them treat their lowest level class as the FC, which combined with extra skill points and a free feat at character creation gives them an annoying advantage; this is also a thing for half-elves, which is their only real advantage over elves. Meanwhile, half-orcs get ability adjustments that provide diminishing returns - +2 Strength, but -2 Intelligence and Charisma. Could you play one as a wizard or paladin, maybe a halfling as a fighter? Sure, but between half-orcs getting the shaft in mental abilities and halflings getting -2 Strength you probably wouldn't get much out of it. It's enough to make you wonder if WotC fucking hated the idea that anyone would play against the racial stereotypes they had worked hard to lay down.

In order to play the game, you have to literally buy (and presumably are expected to read) something like about 130 pages of the Players Handbook which are almost all rules. That's as a (basic) player. As a DM, or anyone actually wishing to know the deeper mechanics of the game (i.e. anything besides skill use and combat) will have to read a metric fuck-ton of material and shit, calculating CR's, learning all of the different shite about special combat manoeuvres, level progression, how to create items, rules for specific items, rules for flight and mounted moving, rules for survivng, tracking, hunting, picking your nose and so on. Thankfully you no longer have to buy them since the SRD is free and contains all (19 megabytes of the) rules that are absolutely needed (barring leveling related rules).

Many 2nd edition rules were presented as optional, allowing the DM leeway to experiment with his ideas and his group. Carrying over beloved characters from 1st to 2nd edition was no big deal. 3rd edition made this impossible, and canonically standardized plenty of bad and broken rules that made us all want to climb back up into Lorraine's warm life-giving uterus and ask for forgiveness.

Spells all work differently from one another, so instead of looking up the rules on a type of action, you look up the rules for a specific spell. And then the spell's errata. And the Ask the Sage article about that spell.

No one can even pretend the various classes are balanced against one another. After 10th level or so spellcasters are so powerful and versatile that the average dungeon crawl is cut short when they use a spell or two to redirect a nearby river into the front door, killing everything inside but the skeletons. For comparison, the fighter is about to get his third attack a round! But he'll miss with it.

Classes have never been balanced against each other but this is mostly to do with the fact that the power of casters was kept from AD&D but the drawbacks (slower inital leveling speed greater potential to kill yourself, highly limited spell slots and several things that made spellcasting hell) were removed. Granted this example is a bit exaggerated since a smart DM could just quietly change the dungeon to an undead filled one as a middle finger for trying to cheese it or if home to anyone of magical ability, bounce off a ward.

Even so, casters are WAY more powerful on an individual basis. Just check the Tier System. Casters are nigh-always superior in personal combat (oh, trolls trying to fuck me up? Well, guess I'll just fly straight up a few meters and shoot them dead), and have the ability to handle pretty much everything else (short of traps... Damn rogue-only abilites (they can usually bypass them or deal with them some other way than disabling them, though)) as well. They even have specific fucking spells/powers for doing "whatever the fuck I want" ("Wish" and "Reality Revision" comes to mind). The fact that the Adept, a class made for shitty NPCs in mind and thus supposed to be in every way inferior to the player classes, is STILL a solid and perfectly playable Tier 4, at tier or even ahead of most of the Core SRD melee classes, solely because spellcasting is its primary focus.

Some rules make a lot of sense for the sake of mechanics in combat and gameplay but sound silly in realistic terms... like "the older you get the wiser you get"... and by default the better your sense of sight and hearing become. Silly things like these are often pointed out in Rich Burlew's Order of the Stick, an online comic based on D&D characters.

It's possible for a wizard not to know about magic, a druid not to know about nature and a cleric not know about religion (including his own).

LA (level adjustment) was total bullshit anyway, some things have LA way too high for the power you get and others don't get enough for LA +0 (kobolds with +0 LA despite they have -4 str and -2 con). This led to a lot of homebrew LA +0 variants of races.